I've spent over 15 years in the pharmaceutical industry. Typically clinical trials that I have seen are between 500 and 2000 patients. Some of the clinical trials for the COVID-19 vaccines have had up to 30,000 patients enrolled. There is always a chance that things that don't show up in clinical trials manifest themselves when the drug is issued to tens of thousands of people over the course of years instead of the 1,000 or so in the clinical trial. But when you have 30K people in a vaccine trial, the chances of things showing up that don't show up in the clinical trial are going to be very low. But each vaccine will literally be given to hundreds of millions of people. Generally, the way vaccines work is that they take inactive/"dead" virus particles and inject them to simulate the virus infection which in turn stimulates the body to develop a response to it. It teaches the body to develop a response, kind of like a fire drill - it's so that the body knows what to do when it happens for real. Because the virus particles are inactive/"dead", it is extremely unlikely that it will cause an actual infection. It could easily cause a mild/moderate fever that lasts an hour to a day, but that is the body developing an immune response and not an actual infection. You have to consider the alternative. What is the risk of not getting a vaccine? Both have risks; it's a matter of which risk people are more tolerant of. One of the human behavioral heuristics is that people prefer the known to the unknown. For a majority of people, the known is that they don't know anyone personally who has died or been hospitalized from COVID-19. So their default "known" is that they are and have been fine and the scary unknown is the vaccine. But for those who personally know someone who has died or been hospitalized, that's the "known" and not getting the vaccine is the scary unknown. (I don't know anyone personally but my parents' next door neighbors knew the people from Fond du Lac who went on the Nile River cruise.) I'd be willing to bet that the willingness/unwillingness to get a vaccine is as much if not more correlated with personally knowing someone who has died/been hospitalized than it is with politics.